Diverse nuclear genes and extrachromosomally transmitted factors formally characterized as mutators or as suppressors of mutators in Drosophila ananassae will be cytogenetically analyzed with respect to their modes of action, their interactions with selected mutagens, and their relations to other aspects of chromosome behavior, particularly crossing over in males. The results should have implications for the control of spontaneous mutability in natural populations, the etiology of hybrid dysgenesis and the mechanisms of artificial mutagenesis.